Quantitative T1 mapping of the myocardium is useful clinically in the pre-contrast (native) scenario since an increase in T1 is associated with edema or protein deposition, whereas a reduced T1 is associated with lipid or iron deposition. T1 maps from both pre and post-contrast scenarios can be used to quantitatively estimate the volume of contrast agent in the extracellular space, which is indicative of edema, fibrotic scar, or diffuse fibrosis.
Myocardial T1 parameter mapping typically involves inversion or saturation prepared imaging, followed by a series of readouts to sample the T1 recovery. These readouts occur at a specific cardiac phase over multiple heartbeats. They most commonly involve single-shot scans and are acquired over a breath hold for a single slice location. The introduction of navigator gating allows one to scan during free breathing, thus enabling segmented and/or multi-slice acquisitions.
A challenge with T1 mapping during free breathing is that the T1 for myocardium is to the order of 1 second, so the commonly used inversion-prepared T1 mapping techniques require a delay of 8-10 seconds for full signal recovery before a subsequent inversion pulse. Performing inversion prepared T1 mapping during free breathing thus becomes inefficient because of these long compulsory recovery periods after each inversion pulse. Interleaved acquisitions across multiple slices can be performed to improve efficiency, but such approaches are only more efficient for the acquisition of multiple non-overlapping slices, which isn't practical for some cardiac views. Saturation recovery based T1 mapping resets the magnetization after each saturation pulse so these images can be acquired more efficiently, making the technique more suitable for free breathing approaches.
It is also possible to extend the saturation recovery T1 mapping technique to additionally obtain a co-registered T2 map in the same scan. Quantitative T2 mapping is useful for assessing conditions such as acute ischemia, myocarditis and heart transplant rejection, which alter the myocardial water content and consequently prolong the T2 relaxation times. Simultaneous T1 and T2 mapping may be additionally useful, for example, to estimate blood saturation and hematocrit, or to create synthetic images with any desired T1 or T2 weighting.
Myocardial T2 mapping may be achieved by applying multiple T2 preparation pulses prior to the readout to introduce varying levels of T2-dependent signal. Similar to T1 mapping, single shot readouts over multiple heartbeats can be used to obtain images suitable for pixel-wise fitting. Combined T1 and T2 mapping can be achieved by adding varying T2 preparations between the saturation pulse and the readout, and fitting both T1 and T2 using Bloch equations of the signal evolution. Saturation recovery and T2 preparation has previously been combined to perform simultaneous T1 and T2 mapping, but only in a breath hold scenario.